“Every rose has its thorns”, or so the song goes. Except, it’s not true, as we shall see in a minute.
Thorns are part of a plant’s conventional weaponry, defensive structures designed to protect their tasty parts from those pesky herbivores who want to eat them. Botanists call plants that are spiny spinescent, which I think is a lovely word, although of course the spines themselves can be owie.
Botanists have three different categories for spiny growths, dividing them up by the part of the plant from which they have been formed. So thorns are modified, short branches that grow from axillary buds. You can see them on hawthorn and blackthorn (sloes), among other things. The Holy Thorn at Glastonbury is an unusual variety of Hawthorn, Crataegus monogyna ‘Biflora’, which flowers twice a year.
Spines, on the other hand, are formed from modified leaves or leaf parts. Cacti have spines – the fleshy green bits are stems, and their leaves have transformed into spines. Not only do they protect their precious water supplies from hungry animals, but spines lose less water in the desert heat than leaves would; it’s a dual-purpose adaptation.
Apparently the prickly bits at the edges of holly leaves are also spines. They’re called marginal spines and are extensions of the major veins in the leaf. It’s always fascinating to see how plants have achieved the same end via different means. That’s evolution, that is 😉
And those roses? Well, they have prickles, short and woody outgrowths that arise from epidermal tissue and form irregular patterns within internodes. Not quite as catchy, perhaps, but more correct. And in some species, prickles also have a dual function. If you look at brambles, for example, they have recurved prickles that help them scramble up and over anything that gets in their way:
I am only too well aware of the effectiveness of the blackberry’s weaponry, having been involved in a fairly constant battle with them in my garden. You can read one of my early dispatches from the trenches: Operation Bramble.
There are other types of plant defences. Stinging nettles inject you with a painfully irritating poison when you brush against the special epidermal hairs on their leaves. And many grasses have the ability to take up silica from the soil and store it as phytoliths. These sharp particles are another defence against browsing herbivores – it makes plants hard to eat, and really grinds down the teeth that try to chew through them. There’s also some interesting research from the University of Sussex that suggests that once that silica gets into the digestive system, it has some other nasty effects on animals. And you may have encountered them yourself, if you’ve ever sliced through your skin with a blade of grass.
The pointy bits on the fruits of certain Cyclantera species (this one is C. explodens) are referred to everywhere as spines. But are they spines? Are they really?
This is the second post in my “When Plants Attack” series. The first was on one aspect of their chemical weaponry – allelopathy. More evidence of the evil nature of plants will be forthcoming in due course 😉